Demands for utilization of the alternative energy and the clean energy increases because of the rapid increase in utilization of the fossil fuel. As a way of meeting the demands, the electricity generation and storage field applied with the electro-chemistry is most actively researched.
Currently, a representative example of an electrochemical device using such electrochemical energy is a secondary battery, the use of which increasingly expands.
According to the shape of a battery case, the secondary battery is classified into a cylindrical type battery and a rectangular type battery in which an electrode assembly is placed within a cylindrical type or rectangular type metal can, and a pouch type battery in which the electrode assembly is placed within a pouch type case of an aluminum laminate sheet.
The electrode assembly placed within the battery case is an electricity generation device composed of a stack structure of positive electrode/separator/negative electrode capable of charging and discharging, and classified into a jelly roll type formed by winding elongated sheet-type positive electrode and negative electrode coated with active material, along with a separator being interposed therebetween, and a stack type formed by sequentially stacking a plurality of positive and negative electrodes in certain size, along with the separator being interposed therebetween.
As a step-forward structure of the electrode assembly, a combined form of the jelly roll type and the stack type, i.e., a stack/folding type electrode assembly has been developed, in which a full-cell in a certain unit size with a positive electrode/separator/negative electrode structure or a bi-cell with a positive electrode (negative)/separator/negative electrode (positive)/separator/positive electrode (negative) structure, is folded by using a long and continuous separator film.
Further, in order to enhance the process efficiency of a related stack type electrode assembly and to meet the demands for various types of the secondary battery, a lamination/stack type electrode assembly has also been developed, in which unit cells each having the alternately stacked and laminated electrodes and separators are stacked.
Meanwhile, demands for the lithium secondary battery increased, because it is the secondary battery that has a light weight and high energy density. However, compared to the consumer's continuously increasing request for a high energy density battery, energy density of the lithium secondary battery has not sufficiently met the consumer's request.
In order to solve the problem, an effort has been made to increase energy density by using internal space of the secondary battery. However, the effect is less than sufficient, due to structural limitations of the secondary battery.
Specifically, for the battery form in which a plurality of electrode plates and separators are stacked, despite many efforts to reduce a volume occupied by electrode tabs protruded from the electrode plates and a volume occupied by a coupler connecting the electrode tabs and the electrode leads, these efforts failed to bring about satisfactory effects.
Accordingly, for the battery form in which a plurality of the electrode plates and separators are stacked, new technology that can enhance energy density while maintaining overall structure, is highly necessary.